Circuit arrangement for operating a lamp

ABSTRACT

In an electronic ballast circuit equipped with lamp presence detection means that generate a current flowing through one of the lamp electrodes the detection means are periodically switched off. The power dissipated by the lamp presence detection means is thereby reduced.

The invention relates to a circuit arrangement for operating a lampcomprising

-   -   input terminals for connection to the poles of a voltage supply        source,    -   a circuit part I coupled to the input terminals for igniting the        lamp and for generating a lamp current out of a supply voltage        delivered by the supply voltage source,    -   a circuit part II, coupled to circuit part I, for detecting the        presence of a lamp comprising        -   a circuit part III for generating a current through an            electrode of the lamp,        -   a circuit part IV for detecting the current through the            electrode of the lamp.

Such a circuit arrangement is known. The current through the electrodescan be a DC current or a AC current. The circuit part III can berealized in many different ways. For instance circuit part III maycomprise means for supplying a DC voltage source but may alternativelycomprise a switched mode power supply such as a bridge circuit forsupplying a high frequency AC current to the electrodes. In the lattercase the circuit part III may comprise a transformer equipped withsecondary windings that are coupled to the lamp electrodes. The circuitpart II makes it possible for the circuit arrangement to check whether alamp is actually present before attempts to ignite the lamp are made. Noignition voltage is generated in case the circuit part II detects thatno lamp is present. Thereby damage to components in the circuitarrangement due to high currents and voltages is prevented. Similarly,in case a burning lamp is disconnected from the circuit arrangement orthe electrode of the lamp is broken, the circuit part II detects whethera new lamp is connected to the circuit arrangement or when the lamp withthe electrode failure is replaced by a new lamp. In order to make surethat the new lamp that is connected to the circuit arrangement isimmediately detected and ignited, the circuit part II operatescontinuously. A disadvantage associated with this continuous operation,however, is that the current generated by circuit part III continuouslydissipates power.

The invention aims to provide a circuit arrangement for operating a lampin which lamp presence can be detected at any time and in which the lampdetection consumes only a very limited amount of power.

A circuit arrangement as mentioned in the opening paragraph is thereforin accordance with the invention characterized in that the circuitarrangement further comprises a circuit part V for periodicallyactivating and deactivating circuit parts III and IV.

In a circuit arrangement according to the invention, circuit part III isnot operative all the time but only a predetermined fraction of eachperiod in which circuit part III is subsequently activated anddeactivated. As a result the amount of power dissipation caused by thecurrent generated by the circuit part III is considerably lowered.

On the other hand, since circuit parts III and IV are activated in eachperiod the connection of a new lamp, after a lamp has been disconnectedfrom the circuit arrangement or has become defective, is always detectedwithin the duration of a period. A proper choice of the duration of oneperiod can assure that a new lamp will be detected and ignited in such ashort time lapse that a user will hardly notice any delay.

A preferred embodiment of a circuit arrangement according to theinvention is characterized in that the circuit part V comprises meansfor gradually increasing the amplitude of the current through theelectrode during a first time interval, maintaining the amplitude of thecurrent through the electrode at a substantially constant value during asecond time interval and gradually decreasing the amplitude of thecurrent through the electrode during a third time interval. The gradualincrease and decrease of the amplitude of the current generated bycircuit part III prevents interference and under some conditionsmechanical noise.

A further preferred embodiment of a circuit arrangement according to theinvention is characterized in that the circuit part V is equipped withdelay means for activating the circuit part IV a predetermined delaytime interval after the activation of circuit part III. It has beenfound that, depending on the nature of circuit part III, the currentthat is generated by circuit part III immediately after circuit part IIIhas been activated, does not always only flow through the lampelectrode. For instance if there are parasitic capacitances, part of thecurrent will flow through these parasitic capacitances until they arecharged. Alternatively, in case the circuit part III comprises aswitched mode power supply incorporating for instance magnetics, thesemagnetics have to be saturated to a certain extent before the switchedmode power supply generates a current that actually flows through thelamp electrode. In these cases as well as in case the amplitude of thecurrent through the lamp electrode is gradually increased, a reliabledetection of such a current directly after circuit part III has beenactivated is not possible. In the further preferred embodiment thisproblem is overcome by making sure that circuit part IV is onlyactivated and the current through the electrode is only detected whenthe predetermined delay time interval has lapsed after the activation ofcircuit part III.

An embodiment of a circuit arrangement according to the invention willbe explained making reference to a drawing. In the drawing

FIG. 1 shows an embodiment of a circuit arrangement according to theinvention with a lamp connected to it, and

FIG. 2 shows the shape of the current through an electrode of the lampas a function of time.

In FIG. 1, K1 and K2 are input terminals for connection to the poles ofa voltage supply source. Input terminals K1 and K2 are connected torespective inputs of circuit part L. Circuit part I is a circuit partfor igniting a lamp and generating a lamp current out of a supplyvoltage delivered by the supply voltage source. A lamp La is connectedto output terminals of circuit part I. The lamp La is equipped withelectrodes El1 and El2. III is a circuit part for generating a currentthrough an electrode of the lamp La. A first output terminal of circuitpart III is connected to a first end of electrode El1. A second end ofelectrode El1 is connected to a first end of ohmic resistor R. A secondend of ohmic resistor R is connected to a second output of circuit partIII.

In the embodiment shown in FIG. 1, circuit part III is voltage sourcefor supplying a DC voltage. The current through electrode El1 generatedby the circuit part III therefor is a DC current. It is noted, however,that in alternative embodiments of a circuit arrangement according tothe invention the circuit part III may comprise a switched mode powersupply such as a bridge circuit for supplying a high frequency ACcurrent to the electrodes. In the latter case the circuit part III maycomprise a transformer equipped with secondary windings that are coupledto the lamp electrodes. A current through the electrodes can in thatcase be detected either on the primary or on the secondary side of thetransformer.

The first end of resistor R is connected to a first input terminal ofcircuit part IVa. Circuit part IVa together with ohmic resistor R formsa circuit part IV for detecting a current through the electrode of thelamp. Circuit part III and circuit part IV together form a circuit partII for detecting the presence of a lamp. The second end of ohmicresistor R is connected to a second input terminal of circuit part IVa.Circuit part IVa is coupled to circuit part I. This coupling isindicated by means of a dotted line. Circuit part V is a circuit partfor periodically activating and deactivating circuit parts III and IV. Afirst output terminal of circuit part V is connected to an inputterminal of circuit part III. A second output terminal of circuit part Vis connected to an input terminal of circuit part IVa.

The operation of the circuit arrangement shown in FIG. 1 is as follows.

In case the input terminals K1 and K2 are connected to the poles of asupply voltage source, circuit part I will ignite the lamp connected toit, in case that lamp is not defective, and subsequently duringstationary operation generate a lamp current through the lamp. Circuitpart III generates a current through electrode El1 that has the shapeillustrated in FIG. 2. In FIG. 2 time is plotted in arbitrary unitsalong the horizontal axis and current in arbitrary units is plottedalong the vertical axis. It can be seen that during a first timeinterval Δt1 the circuit part III is activated and the amplitude of thecurrent through the electrode is gradually increased. During a secondtime interval Δt2 the amplitude of the current through the electrode ismaintained at a substantially constant level. Only during this secondtime interval the circuit part IV is activated. Since the current hasits maximum amplitude reliable detection is possible during this secondtime interval. During a third time interval Δt3 the amplitude of thecurrent through the electrode is gradually decreased. During a fourthtime interval Δt4 the circuit part III is deactivated and the currentthrough the electrode is equal to zero. After the fourth time intervalhas lapsed the first time interval starts again.

In a practical embodiment of the invention the duration of one period(being the sum of the first, second, third and fourth time interval) waschosen as 2 seconds while the sum of the first, second and third timeinterval was chosen at 10 ms. As a result the power dissipation causedby circuit part III was reduced more than 200 times when compared with asituation in which a current with an amplitude equal to the maximumamplitude of the current shown in FIG. 3 flows through the electrodecontinuously.

In case the lamp is removed from the output terminals of circuit part I,electrode El1 is removed so that the current path between the outputterminals of circuit part III is interrupted. Since no current flowsthrough it the voltage over ohmic resistor R becomes zero which isdetected by circuit part IVa. Via the coupling between circuit part IVaand circuit part L circuit part IVa prevents the generation of anignition voltage by circuit part I. When a new lamp is connected to thecircuit arrangement, its presence will be detected within one period.Via the coupling between the circuit part IVa and circuit part I thisdetection enables circuit part I to ignite this new lamp and operate it.

1. Circuit arrangement for operating a lamp comprising input terminalsfor connection to the poles of a voltage supply source a circuit part Icoupled to the input terminals for igniting the lamp and for generatinga lamp current out of a supply voltage delivered by the supply voltagesource, a circuit part II, coupled to circuit part I, for detecting thepresence of a lamp comprising a circuit part III for generating acurrent through an electrode of the lamp, a circuit part IV fordetecting the current through the electrode of the lamp, characterizedin that the circuit arrangement further comprises a circuit part V forperiodically activating and deactivating of circuit parts III and IV. 2.Circuit arrangement according to claim 1, wherein the circuit part Vcomprises means for gradually increasing the amplitude of the currentthrough the electrode during a first time interval, maintaining theamplitude of the current through the electrode at a substantiallyconstant value during a second time interval and gradually decreasingthe amplitude of the current through the electrode during a third timeinterval.
 3. Circuit arrangement according to claim 1, wherein thecircuit part V is equipped with delay means for activating the circuitpart IV a predetermined delay time interval after the activation ofcircuit part III.